Electrophoretic display screens have many advantages for electronic reading devices because they are able to provide a thin and non-volatile display. However combining a display screen such as an electrophoretic display screen with a capacitance-based touch sensor presents particular problems because the voltage swings involved can easily couple to the grid of electrodes used for touch sensing. These problems are particularly acute in large screen devices with fine resolution touch sensing, since these involve a touch sensing grid with large numbers of electrodes, which can easily pick up noise. There are special problems associated with capacitive touch sensing displays as compared with, say, resistive touch sensing, because voltages on the backplane can be induced onto the touch sensing electrodes, which in turn causes a change in voltage on the electrode capacitance which simulates the effect of touch (which makes a small change to the electrode capacitance).
We are particularly concerned with display screens in which a touch sensing layer or layers including touch sensing electrodes is located adjacent to, for example, laminated over, an electrophoretic display screen which, in turn, is driven by a backplane behind the electrophoretic medium. In some preferred devices the backplane is fabricated using solution based thin film transistors (TFTs), preferably patterned by techniques such as direct-write printing, laser ablation or photolithography. Further details can be found in the applicant's earlier patent applications, including, in particular, WO 01/47045, WO 2004/070466, WO 01/47043, WO 2006/059162, WO 2006/056808, WO 2006/061658, WO 2006/106365 (which describes a four or five layer pixel architecture) and PCT/GB2006/050265, all hereby incorporated by reference in their entirety. Thus in embodiments the TFTs comprise an organic semiconductor material, for example a solution processable conjugated polymeric or oligomeric material, and in embodiments the display, more particularly the backplane, is adapted to solution deposition, for example comprising solution-processed polymers and vacuum-deposited metals.
We describe an example touch sensing electrophoretic display screen structure later. One potential solution to the problem of noise from the electrophoretic display screen is to interpose a transparent conductive layer between the electrophoretic screen and the projected capacitance touch sensor, but in practice this suffers from problems: a layer of sufficient conductivity tends to tint the display colour and/or diminish its brightness, and the additional electrode plane in effect provides a significant capacitance in parallel with the capacitive touch sensor. Therefore the effective change in capacitance in touching the sensor is reduced, thus reducing the touch sensing sensitivity. Moving the conductive layer away from the touch sensing electrodes reduces this effect but also reduces the effectiveness of the shielding.
There therefore exists a need for improved approaches to implementing a projected capacitance touch sensor on a display screen, in particular on an electrophoretic display screen.